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Abstract
The southernmost permanent population of the fiddler crab Leptuca uruguayensis occurs along the Samborombón Bay (36°22′S, 56°45′W, Argentina), an important feeding site for many bird species, including ruddy turnstones (Arenaria interpres), whimbrels (Numenius phaeopus), grey plovers (Pluvialis squatarola), american golden plovers (Pluvialis dominica) and gull-billed terns (Gelochelidon nilotica). Although all these birds are known to prey on many fiddler crab species worldwide, there is no estimation of their joint predation impacts, probably due to the difficulty in conducting experiments on an appropriate spatial scale. In these situations, computer simulation methods are useful tools. By using Monte Carlo methods and field data, we modeled the decrease of a fiddler crab population due to bird predation. The model found that under current bird occurrences and crab densities, birds do not consume more than 0.03% of the studied fiddler crab populations. Birds only consume more than 10% of the population if crab density is below 0.02 crabs m2, or if bird occurrences are at least 3 orders of magnitude higher than currently observed. Both situations are unlikely, as mean crab density is 140 crabs m2, and bird density is never so high. Furthermore, by monitoring three different fiddler crab patches, we found that bird predation cannot account for temporal density changes, suggesting that other population processes are more important than bird predation. In conclusion, even though fiddler crabs may exhibit strong predator-avoidance behavior, direct lethal effects of bird predation are currently small.
Highlights
Soft-bottom environments such as mudflats and marshes are important sites for biological conservation, largely because they are feeding areas for both local and migratory bird species [1]
Under natural bird occurrences (Fig. 1), the Joint Density Consumption Rates caused by the five bird species feeding together were generally low across all fiddler crab densities (Fig. 2)
The Monte Carlo estimations indicate that the proportion of fiddler crabs consumed by birds decreases rapidly with fiddler crab population density
Summary
Soft-bottom environments such as mudflats and marshes are important sites for biological conservation, largely because they are feeding areas for both local and migratory bird species [1]. The southernmost permanent population occurs along the Samborombón Bay (36°22′S, 56°45′W, Argentina; see [27]), where it is found mainly on exposed wet mudflats from the middle to upper intertidal flats near the fringe of an extensive Sporobolus densiflorus (previously Spartina densiflora) marsh. This area is an important stopover site for many non-tropical birds [28,29,30], where fiddler crabs provide food for several species, including ruddy turnstones (Arenaria interpres), whimbrels (Numenius phaeopus), grey plovers (Pluvialis squatarola), american golden plovers (Pluvialis dominica) and gull-billed terns (Gelochelidon nilotica; see [31]). The global effect of predation on crab population size and sex ratio would depend on both the number and relative occurrence of each bird species
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